There are various types of image diagnosis devices such as an X-ray device, a computed tomography (CT) device, an ultrasonic device, an RI image device, an MRI device, and the like. Among them, the MRI device is a very important measuring device in a clinical practice since it is not harmful to the human body, as compared with other image diagnosis devices, and images characteristics of structural materials in the human body.
The MRI device may obtain tissue parameters such as spin density, T1, T2, a chemical shift, magnetization transfer, chemical exchange saturation transfer, hematocele, spectroscopy, and the like, which are unique information on a living body, and may obtain various biological images through these parameters. However, it is quite difficult for the MRI device to obtain images with fat and water accurately separated from each other since the fat and the water co-exist in living body tissue. The fat and the water cause relaxation time differences of T1 and T2, and artifacts are generated due to an inappropriate contrast or a chemical shift in the existing MRI imaging method depending on sensitivity of an MRI signal. Particularly, since a chemical shift phenomenon is present due to fat and water components, a detailed anatomical form cannot be obtained of a marginal zone in a structure enclosed by the fat.
In order to solve this problem, a method of selectively exciting the fat and the water using a frequency selective radio frequency (RF) pulse is most generally used.
FIGS. 1 and 2 are views describing a principle of a fat saturation (suppression) method using an off resonance RF pulse.
A unit of a chemical shift is represented by ppm (parts per million) and is measured as a relative numeral value. Since a water molecule has a chemical shift of 4.7 ppm and the fat molecule has a chemical shift of 1.2 ppm, the water molecule and the fat molecule have a difference of 3.5 ppm (See FIG. 1). In this case, when an external magnetic field is 1.5 Tesla (a resonance frequency 64 MHz), a frequency difference of 220 MHz corresponding to f=(64 MHz)*(3.5 ppm) occurs. That is, 1H of the water molecule has a larger chemical shift and undergoes a minimum effective magnetic field larger than that of 1H of the fat molecule, such that it has a high frequency.
A chemical shift selective imaging sequence (CHESS) method is a method of unilaterally suppressing a signal of any specific frequency using the RF pulse. Since the RF pulse used in this method has only the specific frequency of a measurement tissue, the frequency RF pulse is given to selectively excite only the water or fat, thereby allowing a signal having only one component to be obtained.
The CHESS method of selectively applying a saturation RF pulse that is in accordance with the resonance frequency of the fat will be described with reference to FIG. 2. When magnetization of the water or the fat is selectively excited to be put on an X-Y plane, a spoiler gradient is applied to disperse an X-Y component of the magnetization of the fat, thereby canceling the magnetization signal of the fat effectively. Since the next imaging RF pulse has an influence on only the magnetization signal of the water, only the magnetization signal of the water is generated in the obtained image.
FIG. 3 shows a preparation pulse sequence generally used in a fat suppression method. As shown in FIG. 3, an RF pulse 310 of a very small frequency band is applied in order to selectively excite the fat, and a rewinder 320 and a spoiler 330 gradients are used in order for phase recovery and fat magnetization dispersion.
However, in the fat suppression method, the degree of fat suppression may appear differently due to inhomogeneity of a main magnetic field (BO) at a local portion, and inhomogeneous fat saturation may be caused at the time of performing a test in a zone that is out of the center of the main magnetic field.
A Dixon method is used as another solution, which is a method of suppressing the fat by obtaining two different images using a phase by a processional motion frequency difference between the water and the fat molecules and performing addition and subtraction on the two images, and requires a long period of time due to a post-processing process of reconfiguring the two images. In addition, also in this method, the degree of fat suppression appears differently due to the inhomogeneity of the main magnetic field. Therefore, existing fat saturation methods have a large limitation in obtaining a homogenous image.
A basic principle of an iterative decomposition of water and fat with the echo asymmetry and least squares estimation (IDEAL) method is to mainly separate signals from each other by the phase difference between fat and water signals. In this method, which converts the 2-point Dixon method that has been conventionally used into a 3-point method, the respective echoes in three different phases (water-fat phase shifts −π/6, π/2, and 7π/6) is obtained using a phase difference depending on a difference in a resonance frequency between the fat and the water, and the fat signal and the water signal are separated from each other by a reconfiguration algorithm based on the echoes to generate independent water and fat suppression images. That is, the respective echoes are obtained in three different phases per time echo (TE), and four images such as a water-only image, a fat-only image, an in-phase image, and an out-of-phase image are reconfigured by the reconfiguration algorithm based on the respective echoes. In the IDEAL method, the images are reconfigured based on the signals obtained by performing excitation three times, such that a signal-to-noise ratio is increased. However, in the IDEAL method, test time and reconfiguration time of the images are higher, compared with the existing fat saturation method. Further, since the IDEAL method is also based on the Dixon method, the degree of fat suppression in each region of the human body appears differently due to the inhomogeneity of the main magnetic field.
Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from the hydrogen nuclei of water that has restricted motion to the hydrogen nuclei of water that moves with many degrees of freedom. The water with restricted motion is generally conceived as being bound to macromolecules through a series of hydrogen bonds. Saturated bound spins are excited using the off resonance RF pulse having a small frequency band so as to exchange energy by an interaction with free water spins. An effect of the magnetization transfer may be used to distinguish the articular cartilage, an adjacent joint liquid, synovia in which an inflammation is present, and the like. A physical model for the magnetization transfer as described above may be evaluated as a technical development using an advantage of a magnetization transfer contrast (MTC) image. The magnetization transfer contrast (MTC) image is an image with an increased contrast obtained by radiating the off resonance RF pulse having a continuous wave motion to saturate the resonance RF pulse in a partially restricted pool (See FIG. 4).
FIG. 5 shows examples of an RF pulse and gradient magnetic fields generally used in magnetization transfer. An off resonance RF pulse 510 having a small frequency band and spoiler gradients 530 are used, and charges of the spoiler gradients 530, magnitudes of the spoiler gradients 530, and axes on which the spoiler gradients 530 are applied are determined by an experimental value. However, generally, the influence of the spoiler gradient in imaging methods used in the magnetization transfer is not significant.
Since the off resonance RF pulse is also used in the magnetization transfer method, a post-processing process for artifacts generated due to the inhomogeneity of the main magnetic field is required.
A new technology known as chemical exchange saturation transfer (CEST) may provide a significant new tool for MR molecular imaging. CEST exploits the ability of Nuclear Magnetic Resonance (NMR) to resolve different signals arising from protons on different molecules. By selectively saturating a particular proton signal (associated with a particular molecule or CEST agent) that is in exchange with surrounding water molecules, the MRI signal from the surrounding bulk water molecules is also attenuated. Images obtained with and without the RF saturating pulse reveal the location of the CEST agent. The chemical exchange must be in the intermediate regime where exchange is fast enough to efficiently saturate the bulk water signal but slow enough that there is a chemical shift difference between the exchangeable proton and the water proton resonances. The magnitude of the CEST effect therefore depends on both the exchange rate and the number of exchangeable protons.
The CEST method has advantages over traditional molecular imaging techniques. The image contrast is controlled with radio-frequency (RF) pulses and can be turned on/off at will. The endogenous molecules of interest, in some cases, can be directly detected, eliminating the need for contrast agent to be delivered to, and to specifically react with, the molecule of interest.
Referring to FIG. 6, it may be seen that the magnetization transfer imaging method is used twice in the CEST imaging method. Two off resonance RF pulses having a small frequency band are used in frequencies having opposite signs to obtain the signals. The saturated bound spins are excited in the respective frequencies to exchange energy by an interaction with free water spins, and the chemical transfer amount can be calculated based on the ratio between these signals.
In the CEST method, as shown in FIG. 7, an RF pulse 710 and spoiler gradients 730 are used, and signs of the spoiler gradients 730, magnitudes of the spoiler gradients 730, and the axes on which the spoiler gradients 730 are used are determined by an experimental value. However, generally, the influence of the spoiler gradient used in the CEST method is not significant, similarly to the influence of the spoiler gradient in the imaging method used in the magnetization transfer.
In addition, since the magnetization transfer method is used in the CEST method, the off resonance RF pulse should be used. When the off resonance RF pulse is used, the post-processing process of canceling artifacts generated due to the inhomogeneity of the main magnetic field is required.
FIG. 8 shows a pulse sequence used in an imaging method using an off-resonance RF pulse and a pre-saturation RF pulse.
In this imaging method, as shown in FIG. 8, the pre-saturation RF pulse 805 is used before the off resonance RF pulse 810 is applied. Here, since spins excited by the pre-saturation RF pulse 805 are excited by the off resonance RF pulse 810 once again, signals are returned, such that there occurs a phenomenon wherein undesired signals overlap with the ultimately-obtained signals. This phenomenon occurs in the case in which a frequency present within a frequency bandwidth of the pre-saturation RF pulse is excited using the off resonance RF pulse.
In the fat saturation imaging method, the magnetization transfer imaging method, and the CEST imaging method, the frequency bandwidth is narrow, and the excited frequency changes depending on the frequency of the free water proton and magnitude of the main magnetic field. However, in most cases, since the frequency present in the frequency bandwidth of the pre-saturation RF pulse is excited using the off resonance RF pulse, a problem occurs due to an interference signal by the pre-saturation RF pulse.
The CEST method has advantages as compared with traditional molecule imaging technology. The image contrast can be adjusted or controlled depending on a high frequency applied from the outside. An endogenous molecule of interest may be directly detected without use of a contrast media reacting to the endogenous molecule of interest. However, since the magnetization transfer method is also used in the CEST method, the off resonance RF pulse should be used. When the off resonance RF pulse is used as described above, the post-processing process of canceling artifacts generated due to the inhomogeneity of the main magnetic field is required.
Recently, methods of implementing the CEST method using a spin-lock method have been suggested and have been used in an image emphasizing T1rho value used in muscular skeletal disease. However, since three off resonance RF pulses are used in the spin-lock method, there is a problem that the specific absorption rate (SAR) value is increased, which is a measure of the rate of energy absorption in a living body.